In recent years, as the number of mobile terminal users increases, the number of terminals which are present in the same base station area or the same region is increasing. For this reason, for example, a speech communication disable state may occur around the places in which events that attract large crowds such as fireworks events and coming-of-age events are held. If many users which are present in the same area try communication at the same time, congestion of a communication network occurs, so that it is difficult to connect to a communication network. It is desirable that a mobile terminal can perform communication any time anyplace, but it cannot be realized due to the congestion described above.
FIG. 11 is a sequence diagram illustrating the flow of processing until a mobile terminal A as a mobile terminal of a calling party connects with a mobile terminal B as a mobile terminal of a called party. As illustrated in FIG. 11, when a dial call origination for connecting with the mobile terminal B is acquired from the mobile terminal A (step S01), a network control device A pages the mobile terminal B (step S02) and transmits a ring back tone to the mobile terminal A (step S03). For example, the network control device A is a radio network controller (RNC) which centrally controls a plurality of base stations. When the mobile terminal B does not perform speech communication but is in a connectable state, the network control device A receives a response which represents that speech communication is possible from the mobile terminal B (step S04) and connects the mobile terminal A with the mobile terminal B (step S05). Therefore, the mobile terminal A and the mobile terminal B enter a speech communication enable state.
In the above-described processing, when the network control device A is in a congestion state, the network control device A transmits a busy tone which states “The line is busy. Please call again later” to the mobile terminal A. Therefore, a user of the mobile terminal A can recognize that speech communication with the mobile terminal B is impossible.
A conventional network control as described above is disclosed in for example Japanese Laid-open Patent Publication No. 3-188720.
However, even when the mobile terminal moved to a base station area of a different network control device which is not in a congestion state during transmission of the busy tone, the user of the mobile terminal has to terminate the connection with the network control device and then perform a dial call origination again, and thus it is inconvenient.
FIG. 12 is a sequence diagram illustrating the flow of processing when a mobile terminal of a calling party which is receiving the busy tone is handed over to a different network control device. As illustrated in FIG. 12, when a mobile terminal A performs a dial call origination (step S11), a network control device A which is in the congestion state transmits the busy tone to the mobile terminal A (step S12). Therefore, a user of the mobile terminal A can recognize that the communication network is in the congestion state.
For example, if the mobile terminal A moves from a base station area managed by the network control device A which is in the congestion state to a base station area managed by a network control device B, handover processing is performed between the network control device A and the network control device B (step S13). The handover represents that a network control device which connects with a mobile terminal is changed.
At this time, the network control device B which is connected with the mobile terminal A by a handover continuously transmits the busy tone to the mobile terminal A regardless of whether or not an own device is in the congestion state, that is, even when the network control device B is not in the congestion state (step S14). When the mobile terminal A performs an on-hook operation (step S15a), or when the network control device B performs cut-off processing due to a time out of the busy tone transmission time (step S15b), the connection between the mobile terminal A and the network control device B is cut off.
As described above, in the conventional system, even in the case in which the mobile terminal moves and performs a handover to the network control device which is not in the congestion state during transmission of the busy tone, the network control device continuously transmits the busy tone. Therefore, when encountering the congestion state, the user of the mobile terminal has to repeat action of cutting off a communication request to the network control device, moving to an appropriate place and checking whether or not the mobile terminal can connect with a network, and thus it is inconvenient.